Lumens ---> Candlepower

Has anyone out there done a test to see what a lumen equals in CP ? Every time my Surefire rep stops by, he tells us "there's no way to convert lumens to CP." They might measure there lights with a different machine, but there has to be a way to test them. I am just trying to see if maybe anybody has done an approximate test to translate lumens into CP.

I can not be done. If you take a P60 and measured its candle power in the standard reflector it would be X (or for the sake of argument let's say 10,000 candle power). Now take the same lamp, same lumens and use a reflector that is twice as tight now the candle power would be y (or lets say 18,000 candle power b/c it would be a higher #) thus there is no way to convert the two. Hope this makes sense.

Your surefire rep is correct, you cannot convert between the two, they are measures of different things.

Lumens measure the total light output in all directions. Candlela/candlepower measures the brightness of only the very brightest spot. Two lights of the same lumen rating could have vastly different candlepower ratings if ine has a much narrower beam than the other. Similarly, two lights with the same candela rating could have vastly different Lumen ratings for the same reason.

Only by knowing the exact distribution of the light in each and every beam, could you begin to construct a formula to convert between the two. Since flashlight beams vary so much - even Surefires, trying to construct an approximation to cover most of them would be largely useless. There are too many factors that need to be taken into account, and too many precise measurements that need to be made, to make a practical Lu/CP translation possible.

Which is a shame, but that`s just the nature of the two quantities.

Or take a 3D mag at tight focus the candle power would be a lot more than on its widest setting, however the lumens would remain the same.

Originally posted by Chris M.:
Your surefire rep is correct, you cannot convert between the two, they are measures of different things.

Lumens measure the total light output in all directions. Candlela/candlepower measures the brightness of only the very brightest spot. Two lights of the same lumen rating could have vastly different candlepower ratings if ine has a much narrower beam than the other. Similarly, two lights with the same candela rating could have vastly different Lumen ratings for the same reason.

Only by knowing the exact distribution of the light in each and every beam, could you begin to construct a formula to convert between the two. Since flashlight beams vary so much - even Surefires, trying to construct an approximation to cover most of them would be largely useless. There are too many factors that need to be taken into account, and too many precise measurements that need to be made, to make a practical Lu/CP translation possible.

Which is a shame, but that`s just the nature of the two quantities.

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<font size="2" face="Verdana, Arial">Well said, you would also have to take into account the material used in the reflector. Meaning how well it reflects the light ie. A smooth 5 degree reflector would have a higher candle power than an 5 degree orange peel, with the same lamp.

I usually do the following to arrive at a very rough estimate of lumens given candlepower:

1. Find out the beam's hotspot spread i.e. the angle of the hotspot part of the beam (I usually assume roughly 5 degrees but you can easily measure it)
2. Compute the following: 0.00024 * square of beam spread. Another way to get this number is to shine the beam at the wall from 1 meter away. Measure the radius of the hotspot in meters and calculate: square of radius * 3.1 (Edited).
3. Multiply this by the candle power. This is the lumens in the hotspot. This assumes the hotspot is more or less uniform (not exact but good enough). This probably works best for dimpled or textured reflectors. Shiny reflectors may have larger error.
4. Multiply the lumens by 1.25. This should give you a rough estimate of the lamp lumens. This assumes a 90 degree unfocused spillage from the bezel.

For example a 15000 cp flashlight with a 5 degree beam would translate to 110 lumens give or take 15 lumens or so.

Cheers.

Thanks AGAIN for all the help. I have had many customers put the same question before me and I always told them that there is not currently an official way to convert the two, but I never knew for sure if there was or not. But, I knew I could depend on CPF once again to lead me in the right directions. Have a great holiday season everyone !

Monanza,

Good approximation, but it may be a hair high.

My N2 in SRTH turbohead provides 15000 cd in about a 6x9 inch circle at ten feet. That translates to a full beam angle of about 3.5 degrees. There is no secondary spill. The N2 is rated as a 105lumen lamp IIRC correctly.

Also, I just measured an UltraStringer and it's about 16,000cd (cp) not the 75,000 advertised at least with several attempts at focusing the tightest hotspot.

So you've got to use real, measured candela (candlepower) not advertising candlepower.

Cheers,

Richard

Thanks Richard.

When I find my light meter I'll do some tests on a couple of bezels and see if I can't improve the estimate. It all depends on what the hotspot looks like and what one uses as its diameter/angle. For the N2 with SRTH looks like the approximation overestimates the lumens by 35% . Perhaps some of that is due to fresh batteries? I have not seen the SRTH output to know what the hotspot looks like.

Is your measurement for the Ultrastinger at 10ft?

Cheers.

Hi, Monanza,

Yes, my measurements for most of the bigger lights are at ten feet--sorry I didn't mention it.

Footcandles at ten feet * 100 = candela.

I did go out to 73 feet (as I posted in the US thread) and did not notice a substantial difference. If anything the SRTH was brighter!

Cheers,

Richard

Here is a link. Hopefully it helps:

Surefire flashlight faq